In adult humans, each of over a trillion neurons connects with over a thousand target cells (Tessier-Lavigne, M. et al. (1996) Science 274:1123-1133). These neuronal connections form during embryonic development. Each differentiating neuron sends out an axon tipped at the leading edge by a growth cone. Aided by molecular guidance cues, the growth cone migrates through the embryonic environment to its synaptic target.
Axon growth is guided in part by contact-mediated mechanisms involving cell surface and extracellular matrix (ECM) molecules. Many ECM molecules, including fibronectin, vitronectin, members of the laminin, tenascin, collagen, and thrombospondin families, and a variety of proteoglycans, can act either as promoters or inhibitors of neurite outgrowth and extension (Tessier-Lavigne et al., supra). Receptors for ECM molecules include integrins, Ig superfamily members, and proteoglycans. ECM molecules and their receptors have also been implicated in the adhesion, maintenance, and differentiation of neurons (Reichardt, L. F. et al. (1991) Ann. Rev. Neurosci. 14:531-571).
Olfactomedin is a major protein component of the ECM of olfactory neuroepithelia (Yokoe, H. et al. (1993) Proc. Natl. Acad. Sci. USA 90:4655-4659). Olfactomedin from the bullfrog, Rana catesbeiana, is a 448 amino acid, tissue-specific glycoprotein which contains cysteines that form disulfide bridges and several N-linked glycosylation sites. Ofactomedin forms homopolymers held together by these disulfide bridges and N-linked carbohydrate interactions. These polymers constitute the primary architecture of the olfactory ECM. Yokoe et al. (supra) propose that olfactomedin may influence the growth and differentiation of chemosensory cilia of olfactory tissues.
Four olfactomedin-related rat glycoproteins found exclusively in neuronal tissue are produced from a single gene, designated 1B426b, by a combination of differential promoter utilization and alternative mRNA splicing (Danielson, P. E. et al. (1994) J. Neurosci. Res. 38:468-478). The four mRNAs are related to one another by a shared middle (M) region and by two pairs of alternative 5' (A and B) and 3' (Y and Z) regions. The mRNAs encode glycoproteins of 125, 153, 457, and 485 amino acids designated AMY, BMY, AMZ, and BMZ, respectively.
All four 1B426b mRNAs are found in most regions of the rat brain and are enriched in cortex and hippocampus. From in situ hybridization of brain tissue sections, 1B426b mRNAs were found in neuronal tissue but not in brain white matter indicating that 1B426b expression is neuron-specific. Furthermore, in the pituitary, only the A-type mRNAs are detected; and in adrenal glands, only the B-type mRNAs are found. During rat brain development, A-type mRNAs are first detected in day 16 embryos; B-type mRNAs appear later. The four mRNAs increase gradually until postnatal day 20 when the amount of B-type mRNAs equals the amount of A-type mRNAs.
The Z-domains of the rat AMZ and BMZ proteins show significant sequence similarity with bullfrog olfactomedin. AMZ and BMZ contain several N-linked glycosylation sites and potential cysteine-cysteine disulfide bridging sites. The N-terminal sequences of AMZ and BMZ resemble those of signal peptides which may serve to direct them to the neuronal ECM. Danielson et al. (supra) suggest a matrix-related function for the family of 1B426b glycoproteins in neurons and neurosecretory cells.
The discovery of a new neuronal extracellular matrix protein and the polynucleotides encoding it satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in the treatment or prevention of neurological disorders and cancer.